Separation of thin films from transparent substrates by selective optical processing

ABSTRACT

A method of separating a thin film of GaN epitaxially grown on a sapphire substrate. The thin film is bonded to an acceptor substrate, and the sapphire substrate is laser irradiated with a scanned beam at a wavelength at which sapphire is transparent but the GaN is strongly absorbing, e.g., 248 nm. After the laser irradiation, the sample is heated above the melting point of gallium, i.e., above 30° C., and the acceptor substrate and attached GaN thin film are removed from the sapphire growth substrate. If the acceptor substrate is flexible, the GaN thin film can be scribed along cleavage planes of the GaN, and, when the flexible substrate is bent, the GaN film cleaves on those planes. Thereby, GaN lasers and other electronic and opto-electronic devices can be formed.

This is a continuation of application Ser. No. 09/012,829, filed Jan.23, 1998 now U.S. Pat. No. 6,071,795.

GOVERNMENT INTEREST

This invention was made with Government support under Contract No.FDF49620-97-1-0431-05/00, awarded by the Joint Services ElectronicsProgram. The Government has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates generally to the formation of thin films. Inparticular, the invention relates to transferring a thin film from itsgrowth substrate to another substrate.

BACKGROUND ART

Compound semiconductors, such as the III-V semiconductors, are known tooffer superior performance in some special applications, for example,for high-speed and high-temperature electronics and for optical emittersand detectors in particular optical wavelength bands. For efficientsemiconductor operation, it is generally required that the semiconductorbe crystalline, that is, have a regular atomic arrangement. Thetechnologies for the growth of singly crystalline large-scale bodieshave been greatly advanced for silicon (Si), to a reduced extent forgallium arsenide (GaAs), and to a yet lesser extent for indium phosphide(InP). For other compound semiconductors, such as GaN, bulk crystallinesubstrates are not readily available. Sometimes, the unavailabilityresults from an immaturity in the technology. However, bulk crystallineGaN substrates are intrinsically very difficult to grow because of thehigh vapor pressure of nitrogen above molten GaN. For these materials,the usual practicable procedure involves epitaxially growing thecompound semiconductor upon a crystalline substrate of another materialthat is more easily formed into a crystalline substrate, that is,heteroepitaxy.

Gallium nitride (GaN) is a very interesting III-V semiconductor having abandgap corresponding to the required bandgap for blue lasers and otheroptical devices emitting in the blue region of the spectrum.Semiconductor optical emitters in the red, yellow, and even greenportions of the spectrum are known, but blue emitters are not widelyavailable but are greatly desired both for their very short emissionwavelength, enabling a dense, data recording or reading, and also forthe completion of a three-color optical spectrum, thus enabling a fullmulticolor display. An active device based upon GaN needs to beepitaxially grown upon a substrate, but singly crystalline substrates ofGaN or other equally difficult compound substrates are not readilyavailable. The alloy system (Al, In, Ga)N provides bandgap control overthe entire visible spectrum.

In the case of gallium nitride, it has been discovered that GaN thinfilms can be grown on substrates of sapphire, which is a form of Al₂O₃.A plane of the hexagonal crystal structure of GaN is closely matched toa crystallographic plane of sapphire. Foreign growth substrates areknown for other compound semiconductors. High-quality sapphiresubstrates of up to 150 mm diameter are available at high but reasonableprices. Once the GaN thin film has been epitaxially formed over thesapphire substrate, it may be processed into electronic andopto-electronic devices based upon the semiconductive properties of GaN.However, this fabricational approach does not produce the commerciallybest devices. In the case of GaN, the sapphire growth substrateintroduces difficulties in the fabrication process. For example,sapphire cleaves in a basal plane which is perpendicular to thedirection in which GaN epitaxially grows on sapphire. As a result, theGaN/sapphire composite cannot be as easily diced as silicon. The lack ofgood cleavage with GaN on sapphire is particularly a problem with GaNedge-emitting laser requiring a highly reflective (i.e., smooth) endface, typically provided in other materials by a cleaved face. Reactiveion etching has been used to for etching vertical reflector walls, butcontrol of verticality is a problem. Sink et al. have disclosed analternative process in “Cleaved GaN facets by wafer fusion of GaN toInP,” Applied Physics Letters, vol. 68, no. 15, 1996. However, thisprocess requires abrading away most of the entire sapphire substrate, atedious task and one prone to harm the adjacent GaN thin film.

The processes described above do not address the need for a GaNopto-electronic chip to be integrated with an electronic semiconductorchip, most particularly, of silicon.

For these reasons, a number of technologies have been developed todetach thin films of compound semiconductors from their growthsubstrates and to reattach them to other substrates, whether they be ofsilicon, other semiconductors, or non-semiconductive materials.

Yablonovitch originated the technology of transferring GaAs-based thinfilms from a GaAs growth substrate to a silicon substrate. The processis described by Yablonovitch et al. in “Extreme selectivity in thelift-off of epitaxial GaAs films,” Applied Physics Letters, vol. 51, no.26, 1987, pp. 2222-2224 and by Fastenau et al. in “Epitaxial lift-off ofthin InAs layers, Journal of Electronic Materials, vol. 24, no. 6, 1995,pp. 757-760. This same process is described in U.S. Pat. No. 4,883,561to Gmitter et al. In this process, an epitaxial sacrificial layer isfirst grown on the substrate, and then the desired film is epitaxiallygrown on the sacrificial layer. The as-grown film is separated from itsgrowth substrate by selectively etching away the sacrificial layer witha liquid etchant which attacks neither the substrate nor the GaAs thinfilm, thereby lifting off a free-standing film. The free-standing filmcan then be bonded to a substrate of silicon or other material by one ofa variety of methods, as has been described by Yablonovitch et al. in“Van der Waals bonding of GaAs on Pd leads to a permanent,solid-phase-topotaxial, metallurgical bond,” Applied Physics Letters,vol. 59, no. 24, 1991, pp. 3159-3161. The so bonded GaAs thin film canthen be further processed to form devices or circuits that integrate thefunctionalities of GaAs and of the substrate material. Quantum wells andother advanced structures can be grown on the GaAs thin film prior toliftoff. Similar results of bonding InAs thin films onto GaAs substrateshave been reported by Fastenau et al., ibid.

These prior-art processes have not addressed the important compound GaNas well as other non-GaAs compound semiconductors. Further, theprior-art processes rely upon a liquid etchant dissolving from the sides a very thin sacrificial layer between the growth substrate and theepitaxially formed film. Such a separation process is geometricallydisadvantageous and results in separation times that are commercially uneconomic for large-area films.

Kelly et al. have reported an alternative separation process for GaNfilms in “Optical process for liftoff of group III-nitride films,”Physica Status Solidi (a), vol. 159, 1997, pp. R3, R4. In this process,a GaN film is epitaxially grown on a sapphire substrate. The resultantstructure is then irradiated from the sapphire side with an intenselaser beam at a wavelength of 355 nm. This wavelength is within thesapphire bandgap so that the radiation passes through it, but theirradiation wavelength is somewhat outside of the absorption edge ofGaN. As a result, a significant portion of the laser energy is absorbedin the GaN next to the interface. The intense heating of the GaNseparates the gallium from gaseous nitrogen, thereby separating the GaNthin film from the sapphire substrate. It is known that GaN undergoesincongruent decomposition at temperatures above about 800° C.

The process of Kelly et al., however, suffer s difficulties. We observethat the 355 nm radiation of Kelly et al. has sufficient power that theoverlying GaN film is explosively blown away during the laserirradiation, and fracturing of the film often occurs. Obviously, such anexplosive process does not dependably produce uniform films. Even if theexplosive process is acceptable, the area of the high-energy laser beamsrequired for this process is limited. The beam of Kelly et al. has adiameter of 7 mm. If 7 mm portions are being separated, then it isimpossible to obtain film fragments of larger size. It is greatlydesired to obtain larger film segments by a process that is not soviolent.

SUMMARY OF THE INVENTION

The invention may be summarized as a method of transferring acrystalline film from a growth substrate to an acceptor substrate. Thefilm of one composition is grown on a substrate of another compositionhaving an absorption edge at a shorter wavelength than that of the grownfilm. The film side of the structure is then bonded to an acceptorsubstrate. A strong optical beam irradiates the side of the structurehaving the growth substrate with radiation that passes through thegrowth substrate but which is strongly absorbed in the film, therebyweakening the interfacial bond due to localized decomposition of thefilm at the interface. The intensity of the radiation is, however, lowenough to not cause the irradiated area to separate. Preferably, thelaser is raster scanned over an area larger than that of the laser beam.A separation process is performed after the completion of the laserirradiation. In the example of a GaN thin film, the separation processmay include heating the structure to above the melting point of gallium,which is 30° C. Chemical separation processes may also be used.

Alternatively, a sacrificial layer may be grown between the desired filmand the growth substrate. The optical beam absorbed by the sacrificiallayer can then irradiate from the side of either the growth or acceptorsubstrate that is transparent to the optical beam.

The acceptor substrate may be flexible, for example, an elastomericsubstrate. In this case, the film may be laser scribed along cleavageplanes, and then the flexible substrate is bent to cleave the film atdesired facets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the practice of one embodiment of theinvention.

FIGS. 2 through 6 are cross-sectional views of structures producedduring the process of FIG. 1.

FIG. 7 is a cross-sectional view of an extension of the process of FIGS.2 through 6.

FIG. 8 is a plan view of an opto-electronic GaN thin film integratedonto a silicon substrate providing electronic functions.

FIG. 9 is a plan view of a GaN thin film attached to a flexiblesubstrate and scored for dicing.

FIG. 10 is a cross-sectional view of the structure of FIG. 9 bent on amandrel to cleave the GaN thin film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the invention follows the process flow diagram of FIG.1. In step 100, a thin film 102, illustrated in the structural crosssection of FIG. 2, is grown on a donor substrate 104. In a specificembodiment, the thin film 102 is composed of gallium nitride (GaN) to athickness of 3 μm, and the donor substrate 104 is composed ofcrystalline sapphire. It is known that this combination enables thegrowth strongly crystalline GaN, as has been described by Nakamura in“GaN growth using GaN buffer layer,” Japanese Journal of appliedPhysics, vol. 30, 1991, pp. L1705-L1707 and by Nakamura et al. in “Novelmetalorganic chemical vapor deposition system for GaN growth,” AppliedPhysics Letters, vol. 58, no. 18, 1991, pp. 2021-2023. See also thedisclosure by Detchprohm et al. in “The growth of thick GaN film onsapphire substrate by using ZnO buffer layer,” Journal of CrystalGrowth, vol. 128, nos. 1-4, 1993, pp. 384-390. The composite structureof GaN on sapphire is available from CREE Research Inc. of Durham, N.C.having the GaN deposited on a polished side of the sapphire. Because ofthe later laser processing, the backside of the sapphire should also bepolished, for example, with 0.5 μm diamond paper or diamond-containingslurry.

In step 106, a bonding layer 108, as illustrated in the structural crosssection of FIG. 3, is applied either on top of the thin film 102 or onan acceptor substrate 110, and in step 112 the donor substrate 104 isjoined to the acceptor substrate 110 to form a bonded compositestructure illustrated in FIG. 2. It is possible to rely upon van derWaals bonding between crystalline materials in place of a distinctbonding material, as is described by Sink et al. in the previously citedarticle and by Bhat et al. in U.S. Pat. No. 5,207,864. Such a bonding issometimes explained in terms of atomic rearrangement and may begenerically described as fusing together two dissimilar materials.

In the specific embodiment, the bonding layer 108 is Double/Bubble, acommercially available 5-minute epoxy, applied to the GaN thin film 102to a thickness of about 5 μm. Prior to bonding, the backside of thesapphire is polished using diamond paper. The acceptor substrate 110 isa boron-doped, p-type silicon wafer with a <001> crystallineorientation, and the resulting structure is inverted and pressed againstthe acceptor substrate 110 consisting of silicon to form the structureof FIG. 3. Spin-on glass (SOG) or Crystalbond can be used in place ofDouble/Bubble.

In step 114, as illustrated in FIG. 4, a laser beam 116 irradiates thecomposite structure, preferably from the side of the donor substrate 104with radiation that passes through the donor substrate 104 but which isstrongly absorbed by the thin film 102 in a separation region 118 of thethin film 108.

In the specific embodiment, the laser radiation incident upon thesapphire donor substrate 104 may be 248 nm radiation from a KrF pulsedexcimer laser having a pulse width of 38 ns. This radiation easilypasses through the sapphire donor substrate 104 but is strongly absorbedby the GaN thin film 102 in a separation region 118. In this irradiationstep 114, a relatively small laser beam preferably rasters the area ofthe film segment to be separated. The actual irradiation does notseparate the film from its substrate. Because the irradiation processaffects only the buried interface, the irradiation can be performed ineither vacuum, air, or other ambient.

The energy density of the incident beam in an experimental phase wasvaried between 100 and 600 mJ/cm² with the attenuation of the 248 nmradiation in the 0.5 mm thick sapphire estimated to be 20 to 30%.Radiation at 200 mJ/cm² created no visual change. At about 300 mJ/cm²the separation region 118 assumed a metallic silvery color, suggestiveof the decomposition of GaN into metallic gallium and gaseous nitrogen.Multiple pulses at 200 mJ/cm² did not detach the films; however, onepulse at 400 mJ/cm² was sufficient for separation, as described below.

Unlike the process of Kelly et al., the energy density at the interfacein the above described process is not sufficient to separate the growthsubstrate from the acceptor substrate. An entire substrate may be laserprocessed, or a patterned portion may be, prior to the separation.

Wong et al. have disclosed the interaction of 248 nm laser radiationwith GaN in “Pulsed excimer laser processing of AlN/GaN thin films,”Materials Research Society Symposium Proceedings, vol. 449, 1997, pp.1011-1016. However, this work was directed to annealing and dopantactivation of Mg-implanted GaN films, and the irradiation was performedfrom the film side.

After the laser irradiation creating the separation region 118, aseparate process 120 is used to separate the two substrates 104, 110with the film 102 remaining bonded to the acceptor substrate 110, asillustrated in FIG. 5. A residue 122 from the separation layer 118 mayremain on one or both of the substrates. After separation, the growthsubstrate 104 may be reused, a particular advantage when such substratesare expensive.

In the specific example of a GaN film, simple heating of the entiresample to above the melting point of gallium, that is, above 30° C.,melts the gallium in the separation layer 118 without reintegrating thegaseous nitrogen. The residue is believed to be a film of 50 to 100 nmthickness composed of gallium which solidifies when the temperature isreduced to below 30° C.

In step 124, the residue is removed to produce the final bondedstructure illustrated in FIG. 6. The gallium residual film of thespecific example can be removed by a 50:50 volumetric mixture of HCl andH₂O, which does not affect the GaN.

It is also possible to use a liquid etchant that is selective to thematerial of the separation region 118, similarly to the lift-off processtaught in the Yablonovitch references.

If desired, in step 126 the thin film 102 is lifted off the acceptorsubstrate 110 to produce a free-standing thin film 102. If the bondinglayer 108 is an organic wax or glue, such as the Crystalbond or spin-onglass, a properly chosen organic solvent, such as acetone, at the propertemperature will dissolve the glue without affecting the film 102.Alternatively, the bonding layer 108 may be metal with a moderatemelting point, for example, a solder.

Alternatively, in step 128 the transferred thin film 102 can be used asa growth substrate for subsequent epitaxial growth. In the case of thevery high-quality GaN required for lasers, a GaN film 129, illustratedin the cross-sectional view of FIG. 7 is epitaxially deposited over thetop surface of the transferred GaN film 102 after that top surface hasbeen polished and cleaned. A complex laser or other opto-electronicstructure can similarly be grown on the transferred film 102. Nakamuradescribes the fabrication of such a GaN laser in “FirstIII-V-nitride-based violet laser diodes,” Journal of Crystal Growth,vol. 170, 1997, pp. 11-15.

The above process as detailed in the specific example was used totransfer up to 3 mm×4 mm films of GaN having thicknessesf 2.5 to 3 μmonto 5 mm×5 mm silicon substrates. X-ray diffraction tests performed onthe GaN bonded first to the growth sapphire and then to the siliconsubstrate showed an unchanged (0002) GaN reflection at about θ=17.3°with respective full-width half-maxima of 0.0976° and 0.0977°. That is,the crystallinity of the GaN was not affected by the transfer.

It is not necessary to use a laser as the light source as long as thelight intensity is sufficient to form the separation layer 18. Forexample, UV light of sufficient intensity may be used to convert GaN tometallic gallium and included nitrogen bubbles.

The above example used silicon as the acceptor substrate. Thecombination, as illustrated in the plan view of FIG. 8, of a smaller GaNfilm 130 and a larger silicon substrate 132 to which the GaN is bondedis particularly advantageous. The combination allows the integration ofoptical components, such as the GaN laser of Nakamura, in the GaN andelectronic integrated circuitry in the silicon. Bonding wires 134 orother electrical lines interconnect the two portions. Similarly, a GaAsacceptor substrate may be used with GaN. AlInGaP can be epitaxiallygrown on the GaAs, and the combination of the different materials allowsfor the fabrication of arrays of microscopic red, green, and blue lightemitter diodes on a single GaAs substrate for color displayapplications.

The use of silicon, GaAs, InP, and other crystalline materials as theacceptor substrate is further advantageous if the {1,−1,0,0} cleavageplane of the GaN film is crystallographically aligned with a cleavageplane of the acceptor substrate. The structure of the GaN film and thealigned acceptor substrate can then be readily cleaved.

Other types of acceptor substrates may be used. For example, glass orother ceramics may be used. Indium tin oxide (ITO) is known to bond wellwith these materials, and thus may be used as the bonding layer. Waxsuch as Apiezon or Crystalbond be used as a temporary acceptor substratewhich also acts as the bonding layer.

One alternative acceptor substrate is an elastomeric or othermechanically compliant substrate. An example of an elastomeric film isGelPak, available from Vichem Corporation of Sunnyvale, Calif. Metalfoil can also be used. A compliant substrate is particularly interestingfor films of GaN and other laser materials. As illustrated in plan viewin FIG. 9, a GaN film 130 is bonded to a compliant acceptor substrate140. Reactive ion etching (RIE) or milling is used to emboss in the GaNfilm a series of perforations 142 running parallel to the desired{0,1,−1,0} cleavage planes for GaN, and a series of dense or continuousnotches 144 running perpendicularly along the {2,−1 ,−1,0} planes in theregions where cleaved facets are desired. It is not necessary to etchthe notches 144 completely through the film 130, but only enough toinitiate the separation of the film 130 in the plane perpendicular tothe notches 144 and the plane of the film 130. The notches 144 willdelineate the laser dimension perpendicular to the laser facets.

The structure of FIG. 9 is then bent about an axis defined by theintersection of the cleavage plane with the film surface. For example,the structure is conformed to the thick end of a tapered cylindricalmandrel 146 illustrated in axial cross section in FIG. 10, with thedirection of the cleaving perforations 142 aligned along the axis of themandrel 146. The compliant substrate 140 is then slid along the axis toyet smaller mandrel radii until the increased curvature induces the GaNto cleave into axial segments 148 with cleanly cleaved facets along theperforations 142.

The invention is not limited to GaN, but may be used with othermaterials exhibiting incongruent decomposition at elevated temperatures.As mentioned above, the (Al, In, Ga)N alloy family provides tunablebandgaps, advantageous for optical devices. All three of the III-Vcompounds of the alloy, that is, AlN, InN, and GaN exhibit incongruentdecomposition as manifested by their release of nitrogen gas as thetemperature is raised. The II-VI semiconductor ZnO also incongruentlydecomposes. These semiconductors are characterized by their anionforming an elemental gas.

However, AlN, with a bandgap of 6.2 eV, is transparent to 248 nmradiation. Hence, an AlN film grown on a sapphire substrate with anintermediate sacrificial GaN layer can be separated by irradiating theGaN layer from the side of either the film or the substrate. SeparatedAlN films of high crystalline quality, such as could be achieved usingthis process, can be integrated with silicon electronics to fabricatetunable piezoelectric microresonators for gigahertz communicationdevices. Such devices are currently fabricated utilizing poor-qualityAlN films sputtered directly onto silicon at low temperatures so as toavoid undesirable reactions. The technology for microresonators isdescribed by Ruby in “Micromachined cellular filters,” IEEE MTT-SDigest, International Microwave Symposium, IEEE Publication0-7803-3246-6/96, 1996, pp 1149-1152 and by Ruby et al. in “Micromachined thin film bulk acoustic resonators,” Proceeding of the 1994IEEE International Frequency Control Symposium, IEEE Publication0-7803-1945-1/94, 1994, pp. 135-138.

Another example of a material that incongruently decomposes is leadzirconium titanate (PZT) and associated materials, such as leadlanthanum zirconium titanate, and lead niobium titanate. Theseperovskite oxide materials exhibit a variety of behavior, such as beingferroelectric, piezoelectric, etc., and are being developed for sensor,actuator, and memory applications. It is advantageous for someapplications that these perovskite materials be in crystallographicallyoriented forms. It is known that PZT loses PbO at about 600 to 650° C.Thus, in another use of the invention a film of PZT or related materialis grown on a sapphire growth substrate. Then, the growth substrate andPZT film are bonded to an acceptor substrate, and laser irradiation fromthe side of the growth substrate forms a mechanically weak decomposedseparation layer at the PZT/substrate. The included PbO greatly weakensthe bonding, allowing the film to be peeled from the substrate.

Although the above embodiments have been described with the use of laserirradiation, any sufficiently strong optical radiation can be used toform the separation layer.

The invention thus provides a useful and simple method of transferringcrystalline thin films from a growth substrate to an acceptor substrate.It is particularly useful with materials such as GaN which requireheteroepitaxy on substrates that are expensive and difficult to process.

What is claimed is:
 1. A method of separating a thin film from a growthsubstrate, comprising the steps of: providing a crystallographicallyoriented growth substrate with a growth surface and an irradiationsurface; growing a thin film on said growth surface of saidcrystallographically oriented growth substrate; attaching an acceptorsubstrate to said thin film; irradiating said irradiation surface ofsaid crystallographically oriented growth substrate with light of awavelength that is substantially more strongly absorbed in said thinfilm than in said crystallographically oriented growth substrate untilan interfacial layer attaching said thin film and saidcrystallographically oriented growth substrate is formed; aftercompletion of said irradiating step, severing said interfacial layer toseparate said thin film from said crystallographically oriented growthsubstrate.
 2. The method of claim 1 wherein said severing step includesthe step of liquefying said interfacial layer.
 3. The method of claim 1wherein said severing step includes the step of dissolving saidinterfacial layer with ultrasonic energy.
 4. The method of claim 1wherein said severing step includes the step of etching said interfaciallayer.
 5. The method of claim 1 wherein said severing step includes thestep of heating said interfacial layer.
 6. The method of claim 1 whereinsaid attaching step includes the step of applying a glue layer betweensaid thin film and said acceptor substrate.
 7. The method of claim 1wherein said attaching step includes the step of applying a metal layerbetween said thin film and said acceptor substrate.
 8. The method ofclaim 1 wherein said attaching step includes the step of fusing saidthin film to said acceptor substrate.
 9. The method of claim 1 furthercomprising the step of isolating said thin film from said acceptorsubstrate to form a free-standing thin film.